Cloning and expression analysis of an XET cDNA in the peel and pulp of banana fruit ripening and softening

Xyloglucan endotransglycosylase (XET) is thought to be involved in fruit softening through disassembly of xyloglucan, which is the predominant hemicellulose of cell wall. To study the relationship between fruit softening and XET during banana (Musa acuminata Colla cv. Grand Nain) fruit ripening, a full length cDNA (1 095 bp) encoding an XET, MA-XET1, was isolated from ripening banana fruit using RT-PCR and RACE-PCR (rapid amplification of cDNA ends) methods. Sequence analysis showed that the cDNA contains 5' untranslated region of 66 bp, 3' untranslated region of 189 bp and ORF of 840 bp, encoding a predicted polypeptide of 280 amino acids, including DEIDFEFL motif, which is a presumptive catalytic domain conserved in XETs. DNA gel blot analysis demonstrated that MA-XET1 is encoded by a multi-copy family in the banana genome. RNA gel blot analysis revealed that the level of MA-XET1 transcript in the pulp was undetectable, increased and decreased slightly at the preclimacteric, climacteric and postclimacteric stages, respectively. In the peel, accumulation of MA-XET1 transcript was low, increased dramatically and then decreased rapidly, at preclimacteric, climacteric and postclimacteric stages, respectively. Treatment of fruit with propylene, an analog of ethylene, decreased the firmness and enhanced the accumulation of MA-XET1 transcript in the peel and pulp. These results suggest that MA-XET1 is involved in softening of the peel and pulp during banana fruit ripening and its expression is regulated by ethylene at transcriptional level.